Device using glass substrate anodic bonding

ABSTRACT

A bonding technology is disclosed that can form an anodic, conductive bond between two optically transparent substrates. The anodic bond may be accompanied by a metal alloy, solder, eutectic and polymer bond. The first anodic bond may provide one attribute such as hermeticity, whereas the second bond may provide another attribute, such as electrical conductivity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/142,712 (Attorney Docket No. IMT-CoBond) filed Dec. 27, 2013 andincorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

This invention relates to a methodology for bonding together twomicrofabrication substrates.

Microelectromechanical systems are devices which are manufactured usinglithographic fabrication processes originally developed for producingsemiconductor electronic devices. Because the manufacturing processesare lithographic, MEMS devices may be made in very small sizes. MEMStechniques have been used to manufacture a wide variety of transducersand actuators, such as accelerometers and electrostatic cantilevers.

MEMS devices are often movable, they may be enclosed in a rigidstructure, or device cavity formed between two wafers, so that theirsmall, delicate structures are protected from shock, vibration,contamination or atmospheric conditions. Many such devices also requirean evacuated environment for proper functioning, so that these devicecavities may need to be hermetically sealed after evacuation. Thus, thedevice cavity may be formed between two wafers which are bonded using ahermetic adhesive.

Deposition techniques for the thin layers used in semiconductor and MEMSdevices often leave gases incorporated in the layers during deposition.These devices may then be encapsulated in the evacuated cavity forproper functioning. However, the gases incorporated in the films mayescape from the layers during the devices' lifetimes, raising thepressure in the evacuated cavities. Therefore, depending on the degreeof vacuum needed, a gettering material may also be enclosed in thedevice cavity for continuous absorption of contaminant gases.

Accordingly, many designs include such a getter material, which istypically a reactive, metal layer, whose purpose is to absorb thesegases by oxidation, in order to maintain the vacuum levels within thepackage. Because of the reactive nature of these materials, they alsotend to oxidize spontaneously at the surface, forming an oxide layerthat must be removed in order to activate the getter. Activation of thegetter may require exposure to high temperatures, temperaturesconsistent with bonding using glass frit, as described below.

Devices which use or manipulate electromagnetic radiation, such asemitters, reflectors, absorbers, gratings, and the like, may requireencapsulation in an optically transmissive device cavity to functioneffectively. Glass wafers would provide such a cavity. However, ahermetic seal around a glass cavity typically requires a glass fritadhesive, which may require processing temperatures in excess of 400 Cto melt and fuse the frit. Although these temperatures may also beadequate to activate the getter, they may also exceed the temperaturesthat can be withstood by many of the thin metal layers used to createthe optical device. Thus, encapsulation of an optical device in atransparent device cavity which is hermetically sealed has been anelusive goal.

Anodic bonding of a glass substrate to a silicon substrate is known,wherein voltage and heat are applied between the glass wafer and thesilicon wafer. The voltage applied promotes the growth of the oxidelayer between the silicon and the glass, which adheres the materialstogether. However, this method requires one of the wafers be a siliconwafer, which, of course, is not transmissive to most portions of theelectromagnetic spectrum, including the visible portion.

Accordingly, the packaging of optical devices in a hermetic glass cavityremains an unresolved problem.

SUMMARY

Many devices require a transmissive material for the device wafer and/orthe lid wafer, in order for the produced, emitted or alteredelectromagnetic radiation to be transmitted to or from the devicecavity. Infrared emitters, detectors, attenuators, grating and mirrorsfor example, require encapsulation in infrared transmissive materials.Glass is a popular wafer material, however, glass is generally aninsulating material. As a result, anodic bonding between two glasswafers is not possible, because the voltage cannot be applied to theinsulating material properly. Also, it is often desired for the bondingmaterial to be conductive, so as to provide an electrical pathwaybetween structures on either substrate. Therefore a need exists for apackaging technology which can bond two optically transparent substrateswith a conductive bond.

The systems and methods described here provide an electricallyconductive, anodic bond between two optically transparent wafers. Themethod uses ion-rich, nominally conductive optically transparent layerssuch as Borofloat® or Pyrex® glass substrates. The method includes thedeposition of a layer of silicon on one of the optically transparentsubstrate, and the mating of this substrate with a second opticallytransparent substrate to form a substrate assembly. The application oftemperature and voltage to the substrate assembly and the resultantformation of a second oxide layer that bonds the silicon to the secondoptically transparent substrate.

More specifically, the method may include providing a first opticallytransparent substrate and a second optically transparent substrate,depositing a first layer of metal material on the first opticallytransparent substrate, forming a first layer of metal oxide materialwherein the metal oxide is the oxidation product of the metal materialand the first optically transparent substrate, and is formed during thedepositing of the metal material, patterning the first layer of themetal material to create a metal feature, and forming a second layer ofmetal oxide material, wherein the second layer of metal oxide is theoxidation product of the metal material and the second opticallytransparent substrate; and is disposed between the second opticallytransparent substrate and the metal feature, wherein the layer of metalmaterial and the first and second layers of metal oxide form a firstanodic bond between the first optically transparent substrate and thesecond optically transparent substrate.

The resulting device may be a substrate pair assembly including a firstoptically transparent substrate and a second optically transparentsubstrate, wherein the first substrate and second substrate are bondedtogether by a first anodic bond. The first anodic bond may furtherinclude a layer of metal material, and a first layer of metal oxidematerial wherein the metal oxide is the covalently bonded oxidationproduct of the metal material and the first optically transparentsubstrate, The first oxide layer may be disposed between the firstoptically transparent substrate and the metal layer, and a second layerof metal oxide material may be disposed between the second opticallytransparent substrate and the metal layer, wherein the second metaloxide is the covalently bonded oxidation product of the metal materialand the second optically transparent substrate; and is.

The method results in the formation of an anodic, conductive bondbetween two glass substrates. One or more additional bonds may be formedlaterally adjacent to the anodic bond. The additional one or more bondsmay be thermocompression, a polymer, metal alloy, a solder, and aeutectic bond. Examples of appropriate thermocompression bondingtechniques include gold (Au), silver (Ag), or platinum (Pt). and indium(In). The laterally adjacent bond may be selected to provide someadditional attribute, for example like hermeticity, electricalconductivity, low rf loss, high adhesive strength, leak resistance, orthermal conductivity, that the anodic bond may lack.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the followingdetailed description, and from the accompanying drawings, which however,should not be taken to limit the invention to the specific embodimentsshown but are for explanation and understanding only. It should beunderstood that these drawings do not necessarily depict the structuresto scale, and that directional designations such as “top,” “bottom,”“upper,” “lower,” “left” and “right” are arbitrary, as the device may beconstructed and operated in any particular orientation.

FIGS. 1 a and 1 b are simplified cross sectional diagrams of a substratepair assembly wherein two optically transparent glass substrates arebonded anodically to form a device cavity;

FIGS. 2 a and 2 b are simplified cross sectional diagrams of a substratepair assembly wherein two optically transparent glass substrates arebonded anodically and with a second bond spaced laterally from theanodic bond;

FIG. 3 is a simplified cross sectional diagram of a substrate pairassembly wherein the two adhesive layers are patterned on bothsubstrates;

FIG. 4 is a simplified cross sectional diagram of a substrate pairassembly showing a height mismatch between the two patterned features onthe substrates;

FIG. 5 is a simplified cross sectional diagram of a substrate pairassembly showing a height mismatch between a first patterned feature anda second patterned feature;

FIG. 6 is a simplified cross sectional diagram of a substrate pairassembly wherein one of the patterned features is formed of a size toconcentrate pressure on this feature to form the bond;

FIG. 7 is a simplified cross sectional diagram of a substrate pairassembly wherein one of the patterned features is a multilayerstructure;

FIG. 8 is a simplified cross sectional diagram of a substrate pairassembly showing a third bond spaced laterally from the second bond.

FIGS. 9 a and 9 b are simplified profiles of a of bonding conditions,wherein one bond is formed under one set of conditions and the otherbond is formed under the second set of conditions;

FIGS. 10 a and 10 b are simplified profiles of a second set of bondingconditions, wherein one bond is formed under one set of conditions andthe other bond is formed under the second set of conditions;

FIGS. 11 a and 11 b are simplified profiles of a second set of bondingconditions, wherein one bond is formed under one set of conditions andthe other bond is formed under the second set of conditions; and

DETAILED DESCRIPTION

The systems and methods set forth herein are described with respect to aparticular embodiment, that of a wafer pair assembly including a pair ofoptically transparent wafers joined by a first anodic bond. The waferassembly may include a second bond in addition to the anodic bond. Thesystems and methods may be extended, however, to more than two opticallytransparent substrates and using any number of additional bonds usingother bonding technologies. The methods and devices are particularlysuitable with the use of an ion-rich glass such as borofloat Glass®,Pyrex®, both borosilicate glasses, and fused silica are examples ofsuitable materials whose electrical conductivity is acceptably close tothat of silicon. Borofloat glass, manufactured by Schott AG, is a highlychemically resistant borosilicate glass with significant conductivitythat is produced using the float method, and having a composition of 81%SiO2: 13% B₂O3; 4% Na₂O/K₂O; and 2% Al₂O₃. Pyrex, developed by Corning,contains 80.6% SiO₂, 12.6% B₂O₃, 4.2% Na₂O, 2.2% Al₂O₃, 0.04% Fe₂O₃,0.1% CaO, 0.05% MgO, and 0.1% Cl. However, it should be understood thata material having a substantial conductivity should be understood tomean any material whose resistivity is less than about 8 ohm-cm. Itshould be understood that this selection of materials may be applied toall the subsequently described embodiments shown in FIGS. 1-11. As aresult of this selection of materials, the glass portion may conductenough charge to support the application of the applied field and allowthe anodic bond to form.

In the art, the term “wafer” is generally understood to refer to agenerally circular, thin disk of material upon which a plurality ofmicrofabricated devices may be fabricated. The word “substrate” may beunderstood to refer to any supporting surface, such as a wafer forexample, or a wafer after the microfabricated devices have beensingulated. The terms “wafer” and “substrate” have been usedinterchangeably herein, even though wafer generally suggests an intact,circular surface. They should both be understood to mean a supportingsurface on which one or more microfabricated structures have been built.Specifically, “wafer” should be understood to include any portion of theintact, circular fabrication material, including singulated deviceswhich have been separated from the intact, generally circular,fabrication material. FIG. 1 shows the anodic bond, FIGS. 2 and 3 showthe anodic bond in addition to a laterally disposed adjacent bond, FIGS.3-8 show various design options available for the additional bond, FIGS.9-11 show bonding conditions under which the two bonds may be formed.

FIG. 1 is a simplified cross sectional diagram of a substrate pair 100prior to bonding. An upper substrate 110 may have a layer of metal 120formed thereon, wherein the layer of metal 120 is adhered to the uppersubstrate 110 by an oxide layer 130. The oxide layer 130 may be theoxidation product of the metal material 120 with the opticallytransparent substrate 110. For example, if the metal layer 120 issilicon and the optically transparent substrate is glass, the oxidelayer 130 may be silicon dioxide SiO₂ wherein the oxidation has occurredwhile the silicon layer 120 is in contact with the glass substrate, orupon depositing that layer 120.

The metal layer may be any metal capable of forming an oxide with thematerial of the optically transparent substrate 110. Examples ofsuitable metal materials include of titanium (Ti), chromium (Cr),silicon (Si), cobalt (Co), aluminum (Al) and zirconium (Zr), but theremay be others and this list is not meant to be exhaustive.

A suitable deposition method may be sputter deposition using a silicontarget, or chemical vapor deposition. In either case, the result is alayer of metal at least about 500 nm thick and an oxide layer of atleast about 0.3 nm in thickness. It should be understood that this oxidelayer may be made thicker by exposing the structure to heat and/orvoltage, which tends to grow and thicken the oxide until the reaction isquenched by the insulating characteristics of the oxide layer. Afterdeposition, the metal layer 120 may be patterned to form isolatedislands, contiguous lines or other features, or the metal layer 120 mayremain a full film.

A microfabricated device 190 may be formed on the first opticallytransparent substrate 110 or a second optically transparent substrate150. The microfabricated device may be virtually any integrated circuitor MEMS device, but devices which absorb, reflect, transmit, focus, emitor attenuate electromagnetic radiation may benefit particularly from thesystems and methods presented here. In addition to the microfabricateddevice 190, there may be electrical traces (not shown) on either thefirst or the second optically transparent substrate. In someembodiments, it may be necessary or convenient to have electricalconductivity between traces formed on the upper substrate and tracesformed on the lower substrate. The metal layer 120 may provide thisconductivity. An exemplary microfabricated device may be, for example,the infrared emitter described in U.S. Pat. No. 7,968,986, issued Jun.28, 2011 and incorporated by reference herein in its entirety.

With the metal layer 120 and oxide layer 130 formed thereon, the firstoptically transparent substrate 110 may be brought into contact with asecond optically transparent substrate 150 to form a substrate pairassembly 100. Pressure may be applied between the wafers, as well as acombination of heat and voltage. The conditions may be chosen to promotethe formation of a second layer of metal oxide 170 between the metallayer 120 and the second optically transparent substrate 150. This oxidelayer 170 may be thicker, on the order of about 10 nm or more inthickness, compared to the first oxide layer 130. As before, the finalthickness may be a function of the temperature, voltage and pressureapplied to the substrate pair assembly 100, and the duration for whichthese conditions are applied. The oxide layer may be formed in a waferbonding chamber which is equipped to provide these conditions to thewafer pair.

The two optically transparent glass substrates may therefore be bondedanodically to form a device cavity. The first optically transparentsubstrate 110 may be borofloat glass, for example, or any opticallytransparent, suitably conductive substrate material, and the secondoptically transparent substrate 150 may be the same or differentmaterial, but is also nominally conductive.

Upon removal from the wafer bonding chamber, the wafer pair assembly maylook as shown in FIG. 1 b. The anodic bond is formed by the metal layer120 and its associated oxide layers 130 and 170. This anodic bond mayform a perimeter around the microfabricated device 190, enclosing it ina protective device cavity. This anodic bond may have other advantageousfeatures, such as hermeticity, or may provide electrical conductivitybetween the first and the second optically transparent substrates 110and 150.

FIG. 2 a shows a second embodiment of the substrate pair assembly 200.In FIG. 2 a, the anodic bond formed from metal layer 220 has a second,laterally displaced second bond 260 formed also between the substratepair assembly 200. In this and subsequent figures, the number: “1”designated the anodic bond and the number “2” denotes the second bond.The anodic bond may be formed before or after the second bond, so thenumbers: “1” and “2” do not necessarily correspond to the order offormation. The second bond may be laterally separated from the anodicbond 1 by a distance or, for example, about 100 microns or more.

The second bond 2 may be, for example, a polymer, thermocompression,metal alloy, eutectic, a solder, or a metal alloy. Examples ofappropriate thermocompression bonding technologies include gold (Au),silver (Ag), or platinum (Pt). and indium (In). Again, this list is notmeant to be exhaustive. For solder or polymer bonds, the separationbetween the bond lines may be less than 100 microns because the bondingmaterial partially liquefies during the process, making the materialcompliant and spreading it laterally by some amount.

In analogy with FIGS. 1 a and 1 b, the microfabricated device 290 may bedisposed in a device cavity defined by an anodically bonded metal layer220, This metal layer 220 may join a first optically transparentsubstrate 210 to a second optically transparent substrate 250 by virtueof two metal oxide layers 230 and 270. The second bond layer 260 may beformed on either the first optically transparent substrate 110 or thesecond optically transparent substrate 250 or both. The second bondfeatures 260 may be patterned to have a particular shape and dimensions.Although the microfabricated device is not shown explicitly in all FIGS.1-11, it should be understood that a microfabricated device may bedisposed in the device cavity defined by the one or more bondlines.

The second bond 2 technology may be chosen to have different attributesthat the first bond technology. For example, the second bond technologymay be chosen to provide superior mechanical strength, whereas the firstbond technology may be chosen to provide a conductive path between thefirst and the second optically transparent substrate. More generally,the first bonding mechanism may have an attribute selected from thegroup of hermeticity, electrically conductivity, low rf loss, highadhesive strength, leak resistance, thermal conductivity, and the secondbond may provide a second, different attribute chosen from the samegroup.

FIG. 3 shows yet another embodiment, wherein a first opticallytransparent substrate 310 may be joined to a second opticallytransparent substrate 350 by a first anodic bond. The bond may be formedfrom metal layer 320, joined to the first optically transparentsubstrate 310 by a first oxide layer 330 and to a second opticallytransparent substrate 350 by a second oxide layer. The second oxidelayer is not shown in FIG. 3, because substrates 310 and 350 are shownprior to bonding, and the second oxide layer has not yet been formed. Alaterally distant second bond 360 may also join the first opticallytransparent substrate 310 and the second optically transparent substrate350. In this embodiment, all bond features 320 and 360 may be patternedand may have specific dimensions chosen with the followingconsiderations in mind.

In FIG. 3, the components of the first bond 320 are generally of thesame size as the components of the second bond 360. Thus, the surfacesof first bond 320 may make contact at about the same time as the twocomponents of second bond 360. It may be advantageous in some cases,however, to make the components different, as described below withrespect to FIG. 4.

In analogy with FIGS. 1 a and 1 b, the microfabricated device (notshown) may be disposed in a device cavity defined by an anodicallybonded metal layer 320, This metal layer 320 may join a first opticallytransparent substrate 310 to a second optically transparent substrate350 by virtue of two metal oxide layers, a first oxide layer 330 and asecond oxide layer between metal layer 320 and the second substrate 350.The second oxide layer is not shown in FIG. 4, because substrates 310and 350 are shown prior to bonding, and the second oxide layer has notyet been formed. The second bond layer 360 may also join the firstsubstrate 310 to the second substrate 350. Either the first opticallytransparent substrate 310 or the second optically transparent substrate350 or both. The bond features 320 and 360 may be patterned to have theparticular shape and dimensions shown in FIG. 3.

As shown in FIG. 4, it may be advantageous to form the components of thefirst bond 320 to have different shapes compared to its counterpart 362of the second bond. In particular, the thickness of anodic metal layer320 may be different than the thickness of the second bonding layer 362.More specifically, for a given lateral displacement A between the bondlines, the thickness of bonding layers 362 and 366 may be greater thanthe thickness of bonding element 320 by an amount B. Accordingly, thevertical distance B corresponds to the amount of additional travelbetween substrates 310 and 350 before a first contact is made betweenanodic bonding layer 320 and lower substrate 350. As can be seen in FIG.4, upper layer 362 may contact lower layer 366 before anodic bondinglayer 320 contacts the lower substrate 350. Thus, the bonding force on320 may depend on the distance between the first bond line and thesecond bondline, as some of this bonding force is supported by thecompliance of the substrate 310 across the lateral distance between thetwo bondlines. This lateral distance is denoted as “A” in FIG. 4.Generally, for commonly used glass substrates having a thickness ofabout 500 microns, a ratio of A/B of around 50 may be appropriate.Accordingly, in this situation and depending on the compliance of thefirst optically transparent substrate 310, the pressure on the secondset of bonding elements 362 and 366 may exceed the pressure on the firstanodic bonding element 320. For this reason, formation of the secondbond 2 may precede formation of the first bond. 1. However, the order ofthe bonding may be further influenced by conditions in the bondingchamber as described further below.

More generally, the situation may be as shown in FIG. 5. As before,elements 320 and 360 correspond to the elements of the first and thesecond bondlines on the first and the second optically transparentsubstrates, respectively. The anodic element 320 may be thinner (C), thesame thickness as (D), or thicker (E) than the combined thicknesses ofthe first element 362 and the second element 366 of the second bondline360. This relative height may determine the order of the contact of therespective surfaces, but may not determine the order of the bondingmechanisms 1 and 2. In one embodiment, the thickness of the anodic metallayer 320 may be about 50 nm thinner than the components of the secondbondline, such that the components of the second bondline 360 make firstcontact. In this case, the second bondline may be displaced laterallyfrom the first by about 500 nm. The bonding force on the metal bondingelement 320 will then depend on the compliance of the substrate. Inother words, the ratio of height mismatch to offset between thebondlines may be about 1:10 to 1:50, but will depend on the complianceof the substrate 310.

FIG. 6 shows another embodiment of the substrate pair assembly 400. Asbefore, the microfabricated device (not shown) may be disposed in adevice cavity defined by an anodically bonded metal layer 420. Anodicmetal layer 420 disposed on the first substrate 410, and may be bondedto the first substrate 410 by an oxide layer 430. Upon bonding withlower substrate 450, this layer 420 forms a second oxide layer whichbonds the metal layer, and thus the upper substrate 410, to the lowersubstrate 450. Because FIG. 4 shows the substrates 410 and 450 prior tobonding, the second oxide layer is not shown, because the second oxidelayer has not yet been formed. The second bond layer 460 may also jointhe first substrate 410 to the second substrate 450.

Similar to FIGS. 4 and 5, the second bond layer 460 may be formed of afirst feature 462 and a second feature 466 on the first substrate 410and the second substrate 450, respectively. The second bond features 462and 466 may be patterned to have the particular shape and dimensions asshown in FIG. 6. The first optically transparent substrate 410 or thesecond optically transparent substrate 450 may be joined by the adhesiveaction of the first bond 1 and the second bond 2.

In this embodiment, the first feature of the second bonding technology462 is patterned to have smaller dimensions than the feature 420 of thefirst bonding technology and smaller than second feature 466 of thesecond bonding technology. By making the first feature smaller, thepressure applied by feature 462 is commensurately higher than thepressure on the first feature 420 of the first bond 1.

More specifically, for a given lateral displacement A between the bondlines, the thickness of bonding layers 462 and 466 may be greater thanthe thickness of bonding element 420 by an amount F. Accordingly, thevertical distance F corresponds to the amount of additional travelbetween substrates 410 and 450 before a first contact is made betweenanodic bonding layer 420 and lower substrate 450.

If the bonding speed depends only on pressure, the second bond 2 will beformed before the first bond 1. In particular, when the first feature462 contacts the second feature 466, the substrates must deform by anadditional distance F before contact is made between the feature 420 andthe lower substrate 450. Feature 462 may also have a width G which isthinner than the corresponding width H of the second feature of thesecond bonding technology 466. Accordingly, there will exist morepressure on the second bondline 460 in comparison to the first bondline420. All other things being equal, the second bond may precede the firstanodic bond. However, as described in more detail below, otherparameters may be controlled to determine the order of the bonding forsubstrate pair assembly 400.

FIG. 7 shows another embodiment of the substrate pair assembly 500. Asbefore, the microfabricated device (not shown) may be disposed in adevice cavity defined by an anodically bonded metal layer 520. Asbefore, this anodic layer 520 may formed on the first substrate 510 andmama a second substrate 550. This metal layer 520 may join the firstoptically transparent substrate 510 to a second optically transparentsubstrate 550 by virtue of a first metal oxide layers 530 and a secondmetal oxide layer. The second oxide layer is not shown in FIG. 7,because substrates 510 and 550 are shown prior to bonding, and thesecond oxide layer has not yet been formed. The second bond layer 560may also join the first substrate 510 to the second substrate 550. Thesecond bond layer 560 may be formed of a first feature 562 and a secondfeature 566 on the first substrate 510 and the second substrate 550,respectively. The second bond features 562 and 566 may be patterned tohave the particular shape and dimensions as was shown in FIG. 4. Thefirst optically transparent substrate 510 or the second opticallytransparent substrate 550 may be joined by the adhesive action of thefirst bond 1 and the second bond 2.

In this embodiment, the first feature of the second bonding technology562 may be a patterned multilayer 562. The multilayer may have acompliance that is determined by the materials and dimensions of themultilayer 562. This multilayer feature 562 may also be thinner orthicker than the feature 520 of the first bonding technology. By makingthe first feature 562 more compliant, this feature 562 may make firstcontact. Nonetheless, the second bond may not necessarily precede thefirst bond, even though the pressure may be greater as explained furtherbelow.

Examples of the embodiment illustrated in FIG. 7 include any two orthree metals that have a eutectic point of 400 Centigrade or less, suchas silicon/gold, silicon\molydenum, silicon\silver. Alternatively,multilayers which can form a thermocompression bond may be used,including silver\silver, gold\gold, silver\gold, for example. In anotheralternative, low temperature solder bonding materials may be used, suchas indium\gold, indium \silver, gold-tin/gold-copper-silver,indium\copper, antimony-lead/gold-copper-silver, just to name a few.Each layer in the multilayer stack may have a thickness of about 1-10microns or more.

FIG. 8 shows another embodiment of the substrate pair assembly 600. Asbefore, the microfabricated device (not shown) may be disposed in adevice cavity defined by an anodically bonded metal layer 620. Thismetal layer 620 may join a first optically transparent substrate 610 toa second optically transparent substrate 650 by virtue of a first metaloxide layer 630 and a second metal oxide layer. The second oxide layeris not shown in FIG. 8, because substrates 610 and 650 are shown priorto bonding, and the second oxide layer has not yet been formed. Thesecond bond layer 660 may also join the first substrate 610 to thesecond substrate 650. The second bond layer 660 may be formed of a firstfeature 662 and a second feature 666 on the first substrate 610 and thesecond substrate 650, respectively. The second bond features 662 and 666may be patterned to have the particular shape and dimensions shown inFIG. 3. The first optically transparent substrate 610 or the secondoptically transparent substrate 650 may be joined by the adhesive actionof the first bond 1 and the second bond 2.

In this embodiment, a third bonding technology 3 may be employed inaddition to the first two bonding technologies 1 and 2. The thirdbonding technology 3 may be, for example, a polymer, thermocompression,metal alloy, eutectic, a solder, a metal alloy and a eutectic bond. Thefirst feature 682 of the third bonding technology 680 may be a patternedmultilayer or dimensioned as described previously with respect to FIGS.3-7. A corresponding feature 686 may be disposed on the second substrateto form the third bond 680 along with the first feature 682.

It should be understood that the concepts disclosed here may be extendedto any number of additional bonding technologies. Each of thetechnologies may be employed for the same or a different purpose, andmay be selected for the following attributes: hermeticity, electricalconductivity, low rf loss, high adhesive strength, leak resistance,thermal conductivity,

FIGS. 9 a and 9 b are diagrams of a set of bonding conditions which maybe applied in a substrate bonding chamber to determine the order andquality of the bonds formed. Shown in FIG. 9 a is a temperature profileof a heating source that may be applied to the substrate pair assembly100-600, as described previously. The temperature profile may include alower temperature period followed by a higher temperature period, andthese temperatures are plotted as a function of time in FIG. 9 a.Simultaneously to the temperature profile shown in FIG. 9 a, a voltageprofile may be applied to substrate pair assemblies 100-600, as afunction of time as shown in FIG. 9 b. For example, if the first bond tobe formed is an anodic bond, which proceeds at a certain temperature andvoltage, these conditions may be applied in step 1. If a highertemperature is required for the second bond, but no voltage, theseconditions may be applied in step 2. Accordingly, either the anodic bondtechnology or the second bond may proceed first, according to theconditions chosen for each step.

For example, if instead the anodic bond is to be achieved after thesecond bonding, the temperature and voltage profiles shown in FIGS. 10 aand 10 b may be applied. In FIG. 10 a, a ramped temperature function isshown, wherein a first temperature is applied during a first step, and asecond temperature is applied during a second step. However, in thisembodiment, the higher voltage is not applied until the second step,during which the anodic bond may be formed.

Lastly, FIGS. 11 a and 11 b show another embodiment of another set ofbonding conditions. In FIG. 11 a, the temperature profile shown isconstant as a function of time. The order of the bonding in this examplemay be determined by the pressure applied between the substrates asshown in 11 b. For a bonding technology requiring higher pressures butconstant temperatures, this bond is not achieved until the second step.

It should be understood that the bonding parameters of temperature,voltage and pressure may be mixed and matched according to the bondingtechnologies being used. More generally, as illustrated in FIGS. 9 a, 9b, 10 a, 10 b, 11 a and 11 b, a first combination bonding conditions maybe applied in a first step, and a second set of bonding conditions maybe applied in a second step. The bonding conditions may includetemperature, pressure and voltage. Either the first step or the secondstep may achieve the anodic bond. The second bond may be achieved in theother step.

Of course, it should be clear that if the substrate pair assemblyincludes a third or further bonding technologies, a third step or rampmay be included in addition to the two shown.

It should also be clear that any of the concepts described above may bemixed or matched with any or all of the other concepts in terms ofplacement and dimensioning of the bonding features, and control ofbonding chamber parameters.

While various details have been described in conjunction with theexemplary implementations outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent upon reviewing the foregoing disclosure. While the embodimentdescribed above relates to two bonded optically transparent substrates,it should be understood that the techniques and designs described abovemay be applied to any of a number of other materials, includingoptically opaque materials. Accordingly, the exemplary implementationsset forth above, are intended to be illustrative, not limiting.

What is claimed is:
 1. A substrate pair assembly, comprising: a firstoptically transparent substrate and a second optically transparentsubstrate, wherein the first substrate and second substrate are bondedtogether by a first anodic bond, and wherein the first anodic bondfurther includes: a layer of metal material; a first layer of metaloxide material wherein the metal oxide is the covalently bondedoxidation product of the metal material and the first opticallytransparent substrate; and is disposed between the first opticallytransparent substrate and the metal layer; and a second layer of metaloxide material wherein the second metal oxide is the covalently bondedoxidation product of the metal material and the second opticallytransparent substrate; and is disposed between the second opticallytransparent substrate and the metal layer, to form the first anodic bondbetween the first optically transparent substrate and the secondoptically transparent substrate.
 2. The substrate pair assembly of claim1, wherein the first layer of the metal oxide is about 0.3 nm to about0.5 nm thick, and second layer of the metal oxide is up to about 10 nmthick, and the layer of metal material is at least about 50 nm thick, 3.The substrate pair assembly of claim 1, wherein the first and secondoptically transparent substrates comprise ion-rich glass having aresistivity of less than about 8 ohm-cm.
 4. The substrate pair assemblyof claim 1, wherein the metal layer comprises at least one of titanium(Ti), chrome (Cr), silicon (Si), cobalt (Co), aluminum (Al) andzirconium (Zr).
 5. The substrate pair assembly of claim 1, furthercomprising: a microfabricated device, wherein the anodic bond defines aperimeter of a hermetically sealed device cavity formed between thefirst optically transparent substrate and the second opticallytransparent substrate, with the microfabricated device disposed withinthe device cavity.
 6. The substrate pair assembly of claim 5, whereinthe microfabricated device comprises at least one of an emitter,detector, an attenuator and reflector of electromagnetic radiation. 7.The substrate pair assembly of claim 6, wherein the device absorbs,reflects, transmits, focuses, emits or attenuates radiation which passesthrough at least one of the optically transparent substrates.
 8. Thesubstrate pair assembly of claim 1, further comprising: a second bondwhich is laterally adjacent to the first anodic bond.
 9. The substratepair assembly of claim 8, wherein the first anodic bond provides anattribute selected from the group of hermeticity, electricallyconductivity, low rf loss, high adhesive strength, leak resistance,thermal conductivity, and the second bond provides a second, differentattribute chosen from the same group.
 10. The substrate pair assembly ofclaim 8, wherein the second bond is laterally separated from the firstanodic bond by a distance of about 100 microns or more.
 11. Thesubstrate pair assembly of claim 8, wherein the second bond comprisesone of a polymer, a thermocompression, a solder, a metal alloy and aeutectic bond.
 12. The substrate pair assembly of claim 8, wherein thesecond bond comprises a thermocompression bond using at least one ofgold (Au), silver (Ag), or platinum (Pt), and indium (In).
 13. Thesubstrate pair assembly of claim 8, wherein the second bond comprises amultilayer stack, with each layer having thicknesses between about 1 andabout 10 microns.
 14. The substrate pair assembly of claim 13, whereinthe multilayer stack includes at least one of the pairs of silicon/gold,silicon\molydenum, silicon\silver layers.
 15. The substrate pairassembly of claim 13, wherein the multilayer stack is a low temperaturesolder bond, and includes at least one of indium\gold, indium\silver,gold-tin/gold-copper-silver, indium\copper,antimony-lead/gold-copper-silver alloys.
 16. The substrate pair assemblyof claim 13, further comprising a third bond, wherein the third bond islaterally separated from the second bond by a distance of about 100microns or more.
 17. The substrate pair assembly of claim 16, whereinthe third bond is at least one of a polymer, thermocompression, metalalloy, eutectic, a solder, a metal alloy and a eutectic bond.
 18. Thesubstrate pair assembly of claim 16, wherein the third bond comprises amultilayer stack.
 19. The substrate pair assembly of claim 18, whereinthe multilayer stack includes at least one of the pairs of silicon/gold,silicon\molydenum, silicon\silver layers.
 20. The substrate pairassembly of claim 18, wherein the multilayer stack is a low temperaturesolder bond, and includes at least one of indium\gold, indium\silver,gold-tin/gold-copper-silver, indium\copper,antimony-lead/gold-copper-silver alloys.